Interpretive Summary: Although numerous studies have been conducted to accurately deliver chemical droplets on targets, the behavior of chemical droplets after their deposition on leaf surfaces largely remains unknown. Our studies under controlled conditions on the rate of evaporation and residue formation from single droplets amended with or without surfactants or drift retardants and deposited on hairy and waxy geranium leaf surfaces provided answers to this challenging question. This fundamental research has clearly demonstrated the differences in the droplet spread and evaporation process on different fine structures of leaves with different sizes of droplets, chemical formulations and environmental conditions. This research has also clearly advised that the pesticide dosage and spray methods should be recommended differently for different leaf surfaces to obtain the optimal biological effect and reduce pesticide use.

Technical Abstract:
The fate of pesticide droplets on leaves is seriously influenced by spray formulations and fine structures on leaf surfaces. Evaporation times and wetted areas of droplets on hairy and waxy geranium leaf surfaces were determined under controlled conditions. Droplet evaporation processes were taken with stereoscopic sequential images for various droplet sizes, relative humidity (RH) conditions and spray formulations. The spray formulations were combinations of water, a drift retardant, a surfactant, a fungicide and three insecticides. The evaporation time and wetted area of droplets were significantly changed by adding the surfactant or drift retardant to the sprays, but not by adding the fungicide or insecticide. Droplet evaporation times on waxy leaves were longer than those on hairy leaves. Evaporation times increased exponentially as droplet diameter and RH increased with limited variability of regression coefficients independent of spray type and leaf surface. The wetted area of droplets also increased exponentially as droplet diameter increased but it was not significantly affected by RH. On waxy geranium leaf surfaces, the wetted area of pesticide droplets decreased throughout the evaporating process and at all RH conditions, while, on hairy leaf surfaces for the same droplets and at the same RH conditions, the wetted area continued to spread until evaporation was nearly completed. Given that the duration of evaporation time and the extent of the wetted area affect pesticide distribution on waxy or hairy leaves, recommendations for pesticide dosage and spray methods should be specifically customized for different plant species to obtain the optimal biological effect and reduced pesticide use.